1. Field of the Invention
The present invention relates to a semiconductor device suitable for a semiconductor switch for switching an RF signal.
2. Description of Related Art
Existing switches for an RF signal are given below. For example, a PIN diode is used as a single-function switch. Further, a compound semiconductor is mainly used as a multifunctional or high-performance semiconductor switch albeit expensive. It is easy to provide greater functionality to a switch made up of an enhancement type nMOS transistor on an inexpensive Si substrate. However, the switch of this type has a disadvantage in that its performance, especially, isolation characteristic with high frequencies is inferior to that of the compound semiconductor and is required to improve its performance.
FIG. 6 is a circuit diagram showing a conventional switch made up of an enhancement type nMOSFET (hereinafter referred to as nMOSFET). Hereinafter, this semiconductor switch is referred to as Related Art 1. A source 121 and a drain 122 of an nMOSFET 100a are connected with an RF input terminal 111 and an RF output terminal 112, respectively. A capacitor 103 is connected between the source 121 and the RF input terminal 111 for cutting DC components. Likewise, a capacitor 104 is connected between the drain 122 and the RF output terminal 112 for cutting DC components. A backgate 125 of the nMOSFET is grounded. A gate terminal 120 is connected with a terminal for controlling on/off states of a switch (hereinafter referred to as control terminal) 114.
FIG. 7 is a sectional view of the nMOSFET. A diffusion layer 123 serving as a source of an n-type semiconductor and a diffusion layer 124 serving as a drain thereof are formed on the surface of a P-well 126 serving as a backgate of a p-type semiconductor. A gate oxide film 130 is formed above the P-well 126 and between the diffusion layer 123 as the source and the diffusion layer 124 as the drain, and a gate 129 is formed on the gate oxide film 130.
In this case, depletion layers 127 and 128 are formed at a PN junction between the diffusion layer 123 as the source and the P-well 126 and a PN junction between the diffusion layer 124 as the drain and the P-well 126, respectively. Thus, as shown in FIG. 8, in the nMOSFET, the depletion layers 127 and 128 at the PN junction define capacitances 107 and 108 between a source and a drain. Further, capacitances 109 and 110 are formed through the gate oxide film 130 between the source and the drain.
Now, an operational principle of the switch is described. In FIG. 6, when the switch is turned on, a positive voltage not lower than a threshold voltage VT is applied to the control terminal 114. As a result, the gate terminal 120 of the nMOSFET 100a is turned on. Then, an RF signal is output from the RF input terminal 111 to the RF output terminal 112 by way of the capacitor 103, the nMOSFET 100a, and the capacitor 104. When the switch is turned off, the control terminal 114 is set to 0 V. Thus, the transmission of the RF signal through the nMOSFET 100a is controlled by turning off the gate terminal 120 of the nMOSFET 100a. 
Incidentally, as an example of the semiconductor switch, there is an SPST (Single Pole Single Throw) switch that incorporates a shunt circuit and a controlling inverter. FIG. 9 is a circuit diagram showing another conventional semiconductor switch (hereinafter referred to as Related Art 2). The semiconductor switch of this type is disclosed in Japanese Unexamined Patent Application Publication No. 2003-347553, for example.
The circuit includes a switching nMOSFET 100b provided between the RF input terminal 111 and the RF output terminal 112 as well as a shunt nMOSFET 200 provided between the RF output terminal 112 and a ground terminal 118. A control signal for switching between the switching nMOSFET 100b and the shunt nMOSFET 200 is input from the control terminal 114. The control terminal 114 is directly connected with the gate terminal 120 of the switching nMOSFET 100b as in the above case. On the other hand, the control terminal 114 is connected with a gate terminal 250 of the shunt nMOSFET 200 through an inverter 140. Incidentally, when the shunt nMOSFET 200 is turned on, an impedance is set much lower than an impedance of the output terminal 112 (for example, 50 Ω). In addition, the capacitors 103 and 104 for cutting DC components are inserted between the nMOSFET 100b and the RF input terminal 111 and between the nMOSFET 100b and the RF output terminal 112, respectively. A capacitor 105 for cutting DC components is inserted between the shunt nMOSFET 200 and the ground terminal 18.
An operation of the conventional semiconductor switch is described hereinbelow. When the switch is turned off, the control terminal 114 is set to 0 V, and the switching nMOSFET 100b is turned off. At this time, the inverter 140 applies a positive voltage to the gate of the shunt nMOSFET 200 to turn the shunt nMOSFET 200 on.
However, the inventors of this application find the following problems. That is, in the semiconductor switch of Related Art 1, when the switch is turned off, the control terminal 114 is set to 0 V to turn off the gate terminal 120 of the nMOSFET 100a. At this time, as shown in FIG. 8, the capacitances 107 to 110 are formed between the source and the drain. Therefore, the RF signal is unintentionally transmitted through these capacitances. In particular, the depletion layer capacitances 107 and 108 at the PN junction are generally larger than the gate oxide film capacitances 109 and 110 by one order or more, and become big factors behind the leakage of the signal. That is, the nMOSFET 100a of Related Art 1 has a problem in that the input RF signals are not completely blocked and are partially transmitted through the switching nMOSFET 100a. 
In contrast, in the nMOSFET 100b of Related Art 2, the impedance of the shunt nMOSFET 200 is much lower than the impedance of the switching nMOSFET 100b. Hence, the RF signals are mainly transmitted toward the ground terminal 118. Accordingly, the switch including such a shunt circuit can suppress the leakage of the signal to the RF output terminal 112.
However, if a positive voltage is applied to the control terminal 114 when the switch is turned on, the switching nMOSFET 100b is conversely turned on. Then, the shunt nMOSFET 200 is turned off since the gate terminal 250 is set at 0 V. At this time, the input signals are output to the RF output terminal 112 and also leak through the shunt nMOSFET 200 in an off state, leading to a problem in that the output signals are reduced.